Sik (mouse Src-related intestinal kinase) and its orthologue BRK (human breast tumor kinase) are intracellular tyrosine kinases that are distantly related to the Src family and have a similar structure, but they lack the myristoylation signal. Here we demonstrate that Sik and BRK associate with the RNA binding protein Sam68 (Src associated during mitosis, 68 kDa). We found that Sik interacts with Sam68 through its SH3 and SH2 domains and that the proline-rich P3 region of Sam68 is required for Sik and BRK SH3 binding. In the transformed HT29 adenocarcinoma cell cell line, endogenous BRK and Sam68 colocalize in Sam68-SLM nuclear bodies (SNBs), while transfected Sik and Sam68 are localized diffusely in the nucleoplasm of nontransformed NMuMG mammary epithelial cells. Transfected Sik phosphorylates Sam68 in SNBs in HT29 cells and in the nucleoplasm of NMuMG cells. In functional studies, expression of Sik abolished the ability of Sam68 to bind RNA and act as a cellular Rev homologue. While Sam68 is a substrate for Src family kinases during mitosis, Sik/BRK is the first identified tyrosine kinase that can phosphorylate Sam68 and regulate its activity within the nucleus, where it resides during most of the cell cycle.

Breast tumor kinase (BRK) is an intracellular tyrosine kinase expressed in differentiating epithelial cells of the gastrointestinal tract and skin, and in several epithelial cancers including carcinomas of the breast and colon. We examined expression of BRK and its mouse ortholog Srcrelated intestinal kinase (Sik) in prostate tissues and detected it in the nuclei of normal luminal prostate epithelial cells. BRK localization was then examined in 58 human prostate biopsy samples representing various grades of prostate cancer. While nuclear localization of BRK was present in well-differentiated tumors, it was absent in poorly differentiated tumors. However localization of Sam68, a nuclear substrate of BRK/Sik, was unaltered in all prostate tumors examined. Consistent with these results, nuclear BRK was detected in the more differentiated androgen-responsive LNCaP human prostate cancer cell line that is poorly tumorigenic in host animals, but it was primarily cytoplasmic in the undifferentiated androgen-unresponsive PC3 prostate cancer cell line that forms aggressive tumors. While PC3 cells expressed higher levels of endogenous BRK than LNCaP cells, BRK was less active in these cells. Our data suggest that BRK plays a role in differentiation of prostate epithelial cells. Altered BRK localization and/or activity may provide a prognostic indicator for prostate tumor progression and be a potential target for therapeutic intervention.

p53 tumor suppressor is a subject of several posttranslational modi®cations, including phosphorylation, ubiquitination and acetylation, which regulate p53 function. A new covalent modi®cation of p53 at lysine 386 by SUMO-1 was recently identi®ed. To elucidate the function of sumoylated p53, we compared the properties of wild type p53 and sumoylation-de®cient p53 mutant, K386R. No di erences were found between wild type p53 and K386R mutant of p53 in transactivation or growth suppression assays. Moreover, overexpression of SUMO-1 has no e ect on p53-regulated transcription. Biochemical fractionation showed that sumoylated p53 is localized in the nucleus and is tightly bound to chromatin structures. p53 and SUMO-1 colocalized in PML nuclear bodies in 293 cells and the nucleoli in MCF7 and HT1080 cells. However, sumoylation-de®cient p53 mutant showed a similar pattern of intranuclear localization, suggesting that SUMO-1 does not target p53 to subnuclear structures. These data indicate that SUMO-1 modi®cation of p53 at lysine 386 may not be essential for p53's cellular localization, transcriptional activation, or growth regulation. Oncogene (2001) 20, 2587 ± 2599.

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Researchers in the field of tumor suppressor genes are actively attempting to discover new tumor suppressor genes and/or characterize known tumor suppressor genes with the intention of treating and diagnosing cancers. A number of recent patents and patent applications have been published that discuss some of these discoveries. Some of the patents and patent applications discuss newly discovered tumor suppressor genes, including WW Domain-Containing Oxidoreductase (WWOX), Cancer Associated Ring-1 (CAR-1), Human Cervical Cancer Suppressor 1 (HCCS-1), Src-suppressed C kinase substrate (SSeCKS), ADP-Ribosylation factor-like putative Tumor Suppressor gene 1 (ARTS1), and Deleted in Osteosarcoma (DOS). One recent patent describes the discovery that known caspase family member caspase-8 (CASP8) is a tumor suppressor. Another recent patent describes the use of Wilms Tumor suppressor gene (WT1) peptides as a cancer vaccine. In addition, Sakai et al. received a patent describing a fragment of the p51 tumor suppressor gene containing a promoter region, which is useful for identifying compounds that modulate p51 activity. Another patent application recently published describes a chimeric tumor suppressor gene generated by combining a portion of the rat PEG-3 protein with the human GADD34 protein, thus creating a protein with apoptotic activity. These patents and patent applications provide valuable information that may be useful in fighting cancer by focusing on tumor suppressor gene activities.